As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as electric bicycles (E-bikes), electric vehicles (EV), or hybrid electric vehicles (HEV), as well as an energy source for mobile wireless electronic devices, such as mobile phones, digital cameras, personal digital assistants (PDAs), and laptop computers.
A small-sized battery pack having a battery cell packed therein is used for small-sized devices, such as mobile phones and digital cameras. On the other hand, a middle- or large-sized battery pack having a battery pack, which includes two or more battery cells (hereinafter, occasionally referred to as a “multi-cell”) connected in parallel and/or in series with each other, packed therein is used for middle- or large-sized devices, such as laptop computers and electric vehicles.
As previously described, a lithium secondary battery has excellent electrical properties; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and electrical short circuits, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused, with the result that heat and gas are generated, and the high-temperature and high-pressure condition caused by the generation of the heat and the gas accelerates the above-mentioned decomposition. Eventually, fire or explosion may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit for interrupting electric current during overcharge, overdischarge, or overcurrent of the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a bent safety member for interrupting electric current or discharging gas when pressure increases due to the generation of the gas. In the case of a small-sized cylindrical secondary battery, for example, the PTC element and the bent safety member are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical case. In the case of a prismatic or pouch-shaped small-sized secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic case or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety problem of the lithium secondary battery is even more serious for a middle- or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells are used in the multi-cell structure battery pack, the abnormal operation of some of the battery cells may cause the abnormal operation of the other battery cells, with the result that fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle- or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), for protecting the battery cells from the overcharge, the overdischarge, and the overcurrent.
However, as the lithium secondary battery is continuously used, i.e., as the lithium secondary battery is continuously charged and discharged, the generating element and the electrically connecting members are gradually degraded. For example, the degradation of the generating element leads to the decomposition of the electrode material and the electrolyte, by which gas is generated. As a result, the battery cell (the cylindrical or prismatic case or the pouch-shaped case) gradually swells. In the normal state of the lithium secondary battery, the safety system, i.e., the BMS detects the overdischarge, the overcharge, and the overcurrent, and controls/protects the battery pack. In the abnormal state of the lithium secondary battery, however, when the BMS does not operate, a possibility of danger increases, and it is difficult to control the battery pack for securing the safety of the battery pack. The middle- or large-sized battery pack is generally constructed in a structure in which a plurality of battery cells is fixedly mounted in a predetermined case. As a result, the respective swelling battery cells are further pressurized in the restrictive case, and therefore, a possibility of fire or explosion greatly increases under the abnormal operation condition of the battery pack.
Various attempts have been carried out to solve the safety-related problem of a secondary battery. In particular, there has been developed a technology for discharging internal gas of a secondary battery to a predetermined region of a battery case when the secondary battery swells due to overcharge and overdischarge of the secondary battery.
For example, Korean Patent Application Publication No. 2002-49208 discloses a structure in which an open piece, made of a resin material having a melting point lower than that of a sealing part of a pouch-shaped battery case, is included in the sealing part of a pouch-shaped battery case, and, when the internal temperature or the internal pressure of a battery excessively increases, the open piece is melted, whereby the sealing state of the battery is released to prevent the combustion or explosion of the battery.
Also, Japanese Patent Application Publication No. 2001-93489 discloses a structure in which a resin film having a melting point lower than that of a thermal welding layer resin film of a laminate sheet constituting a battery case is included in a portion of a sealing part of the battery case as an inner layer, and the resin film responds to the temperature of a battery at high temperature and high pressure, with the result that the resin film portion, having the low melting point, is softened, melted, and deformed, whereby the sealing part is partially opened, and therefore, gas is discharged from the battery.
However, the above-mentioned technologies need additional components, such as the open piece or the resin film, with the result that manufacturing costs of the battery increases. Also, additional processes are required, with the result that the productivity of the battery decreases. Furthermore, the above-mentioned structures are constructed in a structure in which the predetermined region is opened depending upon the melting point of the predetermined member, with the result that operational reliability is low.
Also, Korean Patent Application Publication No. 2006-112035 discloses a secondary battery including a sealant which is mounted at an electrode lead, in such a manner that the sealant comes into tight contact with a case, for achieving sealing between the electrode lead and the case, wherein the sealant is provided at one side thereof with stress concentration portions which break due to the internal pressure of the case for discharging gas outside. According to this publication, as shown in FIG. 5, the stress concentration portions are formed on an insulative film (sealant) inserted in the sealing part while the stress concentration portions are attached to the top and bottom of the electrode lead, and the stress concentration portions are constructed in the form of a notch or a non-attachment region.
However, when the predetermined notches or non-attachment regions are formed on the insulative film, electrical insulation property of the battery and sealability at the sealing part greatly weaken, with the result that an internal short circuit may occur or external moisture or air is introduced into the battery, whereby the efficiency of the battery lowers. Furthermore, the stress concentration portions easily respond to even a slight amount of gas generated during the normal operation of the battery, with the result that the sealed state of the battery may be released.
Therefore, there is a high necessity for a technology that is capable of guiding internal pressure of a battery cell generated when the battery cell malfunctions in a predetermined direction, while securing the sealability of the battery cell during the normal operation of the battery cell, without using additional components, whereby securing the safety of the battery cell with high reliability.